The present invention relates to a wear-resistant coating for machine parts which are subject to friction wear, such as, in particular, the bearing faces of piston rings used in internal-combustion engines, with the coating being applied by a flame-spraying process, preferably a plasma jet spraying process, in the form of a uniform coating or as a body filling a chamber, or groove.
To improve the service life of machine parts which are subject to extreme wear stresses it is customary to coat such parts with wear protection coatings preferably of metals, metal ceramic materials and/or purely ceramic materials by a flame-spraying or plasma jet spraying process. On the bearing faces of piston rings, molybdenum coatings applied by a flame spraying process or molybdenum containing coatings applied by a plasma jet spraying process have been found particularly satisfactory if they are applied either onto the entire surface area of the bearing faces in the form of a spray coating on a smooth surface or in a groove.
When piston rings are under extreme stresses, as for example when they run dry due to damage to the engine or in modern high-speed Diesel engines, particularly turbo-charged engines, cracks may form in the molybdenum coating as a result of overheating and these may lead to the coatings flaking off or breaking out. For that reason, molybdenum alloys or other alloys as well as ceramic materials with or without the addition of low-melting point alloys or intermetallic compounds have been used primarily as binder metals for the coatings on piston rings. However, only some of these measures have brought the desired results.
Coatings on the bearing faces of machine parts which are subjected to friction wear, in addition to exhibiting good and temperature-stable adhesion to the substrate and a good and temperature-stable cohesion within the coating, must also be, inter alia, burn trace proof and wear-resistant on their surface regions which are in contact with a friction partner, must be porous there to accommodate lubricants, must have sufficient inherent lubrication during dry running and in the start-up phase, and their inherent wear must suffice to adapt them to their counterfaces. Additionally, such coatings should be highly resistant to breakouts and exhibit no or only very slight fatigue even after long periods of use. Likewise, particularly when applied as a spray coating in a groove, the coefficients of thermal expansion of the substrate and of the coating material should be adapted to one another in such a way that stress formations with subsequent coating breakouts cannot develop during engine operation.
The prior art alloy coatings or ceramic coatings, however, usually do not exhibit all these characteristics simultaneously and measures to increase wear resistance of the coatings, for example by the addition of hard metals, have again resulted in reduced adhesion of the layers to the substrate and reduced inherent strength of the coatings so that such coatings are not breakout proof and are sensitive to thermal shocks.
In the piston ring coating art, for example when applying hard chromium electrolytically, it is customary to use intermediate layers to improve adhesion of the hard chromium layer on the substrate or to provide the surfaces with soft metal coatings to improve the start-up behavior of the rings. Although such measures are often effective, the application of intermediate and start-up layers requires additional process steps which increase the cost of the rings and, due to the lack of adhesion between individual layers, this often likewise results in damage under extreme stresses.